Volume of distribution of digital data contents is increasing by leaps and bounds as a result of recent spreading of internet system and starting of ground wave digital broadcasting service mainly offering high vision image programs. Volume of digital data has reached 1,644 peta-byte in 2006 on a worldwide scale and are increasing at a rate of 30% per annum, and 92% thereof is accumulated in digital devices. In the optical disc industry, “heat assist”, “pattern media” and “vertical magnet recording system” are proposed in addition to “holographic system” as technology for data storage exceeding 100 tera-byte/cm2. However, since an important point for practical use is “height of transfer rate”, holographic optical memory is expected as a candidate of a next-generation optical disk.
In the next-generation recording media, page type memory system, especially holographic recording has been proposed as a system replacing conventional memory devices and recently the spotlight of attention is focused upon it since it is a system having high memory capacity and enabling random access and high speed transfer. This holographic recording is explained in detail in some references (for example, refer to Hans J. Coufal, et al. “Holographic Data Storage (Springer Series in Optical Sciences, Vol. 76” Springer-Verlag GmbH & Co. KG, August (2000)).
In this holographic recording system, for example, there are proposed a recording method using a holographic recording medium having transparent substrates arranged at both sides of a holographic recording layer (for example, refer to U.S. Pat. No. 5,719,691) and a recording method using a holographic recording medium having a holographic recording layer provided with a reflection surface at one side thereof (for example, refer to JP2002-123949A).
The basic theory of such holographic recording media is to record data by holographic exposure to change a refractive index of the inside of a holographic recording layer of the media and to reconstruct a change in refractive index in the media, thus reconstructing the data For this holographic recording layer, there are proposed various materials such as a material using an inorganic material (for example, refer to G.B Patent No. 9,929,953), a compound undergoing structural isomerization by means of light (for example, refer to JP10-340479A) and a material utilizing diffusion polymerization of a photo-polymer (for example, refer to U.S. Pat. No. 4,942,112).
In volume holographic material generally using a photo-polymer undergoing photo-polymerization, its holographic recording mechanism is such that upon incidence of interference light in a material, a polymerization initiator is activated in an area receiving high intensity of light, thereby initiating photo-polymerization. In the area receiving high intensity of light, monomers are consumed by the polymerization, and on the contrary, monomers in an area receiving low intensity of light are diffused and transferred toward the area receiving high intensity of light. As a result, a difference in monomer density is said to be generated by a difference in intensity of light, thus resulting in a difference in refractive index and forming a hologram (for example, refer to W. S. Colburn, K. H. Hains: Appl. Opt., 10, 1636 (1971)).
For example, photo-polymers comprising a thermoplastic binder resin, a photo-radically polymerizable monomer and a photo-initiator were proposed by Du Pont and a part of them has been put into practical use.
In these photo-polymers, a hologram is formed by a difference in refractive index between the low refractive index binder resin and the high refractive index photo-polymerizable monomer having aromatic ring, and as a result, hologram performance such as high diffraction efficiency is exhibited.
However, while monomers easily move at exposure of interference light and degree of refractive index modulation is increased, there arises a problem with heat resistance and mechanical properties since a thermoplastic resin having low glass transition temperature is used, and a solvent need be used in order to decrease viscosity of the composition at coating on a substrate. Accordingly, the solvent must be completely removed after the coating, and there is a limit in a thickness of a photo-sensitive film. The thickness is up to about 100 μm.
When the film is thick, in the case of inferior compatibility of the binder polymer with the monomer, there is significant limit in practical use due to remarkable light scattering property and large polymerization shrinkage, which is a problem to be solved.
D. J. Lougnot introduced an example of the use of a monomer mixture on volume hologram material in “Photopolymer Recording Materials for Holography Some recent developments”, SPIE, pp. 10-22 (1993). One of the monomers is a tetra-functional monomer and is high in reactivity, and is polymerized at pre-curing step to form a matrix.
In “Photopolymer Recording Materials for Holography Some recent developments”, SPIE, pp. 10-22 (1993), pre-curing is conducted by direct photo-absorption using deep-UV exposure because of difficulty in preparation due to lack of a photo-initiator necessary for hologram recording. As a result, the obtained holographic medium is formed from a matrix host comprising a photo-reactive polymer and a photo-initiator. A reported maximum thickness of a hologram film is 100 μm.
On the other hand, Japanese Patent No. 3330854 discloses a polyfunctional oligomer/monomer mixture having reactivity of the same level and comprising a polyfunctional oligomer having a polyether trunk chain and a molecular weight of not less than 1,000 for the purposes of forming a thick film without using a solvent and decreasing light scattering property.
The hologram matrix is polymerized in-situ, but is formed by crosslinking the polyfunctional oligomer by setting pre-curing conditions so that a part of mono-functional photo-reactive monomers should remain unreacted, which differs from conventional method. Accordingly, a photo-initiator necessary for hologram recording and 80% of monomers are consumed beforehand at a pre-curing step, thereby leading to a demerit that monomers necessary for actual recording are lacking. Also, there are demerits that a speed of diffusion and transfer of monomers in three-dimensional polymer matrix formed in the pre-curing step is slow and sensitivity thereof is inferior. For that reason, a minimum distance of shift hologram multiplexed recording in the optical data recording medium is up to 3 μm.
When using a fluorine-containing material for a base polymer, in general, commercially available fluorine-containing materials have demerits that lowering of refractive index by introduction of fluorine to the base polymer which is a primary purpose is insufficient due to lowering of compatibility as fluorine content is increased; a lot of monomers cannot be contained; and inferior recording sensitivity and high polymerization shrinkage are caused because of light scattering property due to inferior compatibility and also because the materials have no crosslinkable gel structure. This is described, for example, in Japanese Patent No. 2625028 as that “When fluorine is present in a binder, a refractive index of the polymer is lowered, thereby imparting refractive index modulation property to a film and hologram subjected to image recording. Degree of refractive index modulation increases as the fluorine content increases, but the fluorine content is limited in order not to make the film opaque. Accordingly, the representative fluorine content is within a range from about 3% to about 25% though an effect of fluorine content can be achieved even when the fluorine content is as low as 1%”.
There are known other fluorine-containing polymers (Japanese Patent No. 3370762, JP8-241026A and JP8-272284A), but those polymers have similar demerits. Also, since a specific solvent is used for those polymers when forming a hologram film, a thick recording film cannot be obtained.
Example of using no specific solvent is referred to in JP2003-302726A, in which a fluorine-containing oligomer/monomer mixture having perfluoroether chain is disclosed. However, a speed of diffusion and transfer of monomers in a matrix is slow, and recording sensitivity is lacking.
Accordingly, inherent functions of fluorine are not exhibited.